Malaria is a mosquito-borne infectious disease caused by a parasite. At least four species of malaria parasites can infect humans under natural conditions: Plasmodium falciparum (P. falciparum), P. vivax, P. ovale and P. malariae. The first two species cause the most infections worldwide. P. vivax and P. ovale have dormant liver stage parasites (hypnozoites) that can reactivate (or “relapse”) and cause malaria several months or years after the infecting mosquito bite; consequently, these species can be difficult to detect in infected individuals. Severe disease is largely caused by P. falciparum while the disease caused by P. vivax, P. ovale, and P. malariae is generally a milder disease that is rarely fatal.
In humans, the parasites grow and multiply first in the liver cells and then in the red blood cells. In the blood, successive broods of parasites grow inside the red cells and destroy them, releasing daughter parasites (merozoites) that continue the cycle by invading other red cells. The blood stage parasites cause the symptoms of malaria. When certain forms of blood stage parasites, gametocytes, are picked up by a female Anopheles mosquito during a blood meal, they start another, different cycle of growth and multiplication in the mosquito. After 10-18 days, the parasites are found as sporozoites in the mosquito's salivary glands. When the Anopheles mosquito takes a blood meal from another human, the sporozoites are injected with the mosquito's saliva and start another human infection when they parasitize the liver cells.
Infection with malaria parasites can result in a wide variety of symptoms, typically including fever and headache, in severe cases progressing to coma or death. There were an estimated 225 million cases of malaria worldwide in 2009. An estimated 781,000 people died from malaria in 2009 according to the World Health Organization's 2010 World Malaria Report, accounting for 2.23% of deaths worldwide. Ninety percent of malaria-related deaths occur in sub-Saharan Africa, with the majority of deaths being young children. Plasmodium falciparum, the most severe form of malaria, is responsible for the vast majority of deaths associated with the disease. Children suffer the greatest morbidity and mortality from malaria, yet this age group has not been targeted at the identification stage of vaccine development. Of the 100 vaccine candidates currently under investigation, more than 60% are based on only four parasite antigens—a fact that has caused considerable concern. New antigen candidates are urgently needed.